1. Field of Invention
The present invention relates to a conveyance apparatus. More particularly, the present invention relates to a conveyance arm device which has a function to eliminate or dispel static electricity accumulated during wafer conveyance.
2. Description of Related Art
Static electricity is a major source of wafer damage during the wafer fabrication process and after the wafer is fabricated. People walking on a carpet in a relatively high humidity ambience carry from hundreds to thousands of volts of static electricity. In an ambience with a relatively low humidity, the static voltage on the human body may be over ten thousand volts. When these charged carriers contact with wafers, the static electricity is discharged to the wafers, and thus, damages the wafers. In complementary metal-oxide semiconductor (CMOS) fabricating processes, the problem caused by the static electric discharge is especially serious.
A wafer fab is required to be a clean, dirtless environment, usually with a relatively low humidity. However, a high static electricity voltage is often generated in a wafer fab. Therefore, the equipment for wafer fabrication is usually designed to be conductive, so as to minimize the damage caused by accumulated static charges.
But many fabricating devices are still made of insulators under the consideration of fabrication cost, convenience of operation, and possibility of scratching wafers. It is known that the accumulated static electricity damages the wafers and affects the yield of the products. FIG. 1 shows a conventional conveyance arm device used in a furnace in a wafer fab for conveying wafers. The conveyance arm device comprises a conductive seat 12 and a tweezer 10. One end of the tweezer 10 is connected to the conductive seat 12. The tweezer 10 is vacuumed in order to dispose the wafers onto the tweezer 10 firmly without causing vibration or falling from the tweezer 10 during conveyance. The operation of the conducting seat 12 is controlled to drive the tweezer 10, so that the wafers are conveyed to a predetermined place by the tweezer. Generally, the tweezer 10 that conveys the wafers is made of quartz (SiC) to protect wafers from being scratched. Since quartz is not a conductor, the static charges thereon can not be timely discharged or dispelled. In addition, the wafer boat used to carry the wafers is also made of quartz. Thus, static charges are continuously accumulated, and particles in the ambience are easily absorbed onto the wafers to cause a static voltage as high as 4000V to 10000V; thus, the wafers are easily damaged, and the yield is lowered.